A Control of ENSO Transition Complexity by Tropical Pacific Mean SSTs Through Tropical-Subtropical Interaction

El Niño-Southern Oscillation (ENSO) transitions from one event to another in complex ways. Using observational analyses and forced atmospheric model experiments, we show that a preceding ENSO event can activate a subtropical Pacific forcing mechanism to trigger another ENSO event during the following year. These tropical-subtropical Pacific interactions result in a cyclic ENSO transition if the two ENSO events are of opposite signs or a multiyear ENSO transition if they are of the same sign. The preceding ENSO event should excite deep convections in the tropical Pacific in order to activate the subtropical Pacific mechanism. This requirement enables mean temperatures in the cold tongue and warm pool to respectively control how easily the cyclic and multiyear transitions can occur. A future warmer tropical Pacific is projected to decrease the frequency of occurrence of multiyear ENSO transitions but increase the occurrence of cyclic ENSO transitions.